1. Field of Invention
This invention relates generally to ablation of diseased or excess tissue by delivery of electromagnetic radiation to the target tissue. In this process, the absorption of electromagnetic radiation by various physical mechanisms causes heating sufficient to create necrosis and resorption of the target tissue. The electromagnetic radiation is often but not exclusively delivered by some form of antenna.
2. Background of the Invention
Conventionally "microwave" radiation is the designation for radio energy at frequencies from about 300 Megahertz (MHz) to about 30,000 MHz. In this application, the term "microwave" refers to that frequency range and the term "radio-frequency" (RF) is used to describe electromagnetic radiation at frequencies below that level.
Systems for tissue ablation in diverse circumstances using RF and microwave energy are or will very soon be available commercially. Two such systems for the treatment of prostate enlargement are known.
Benign Prostatic Hyperplasia (BPH), non-malignant tissue growth in the prostate gland anterior to and lateral to the urethra, is the most common affliction of the internal organs in men and is a significant problem for a majority of men over the age of about 50. BPH growth normally begins away from glandular tissue posterior to the urethra, which remains healthy. The benign tumorous tissue growth results in narrowing of the canal through which the urethra passes and consequent pressure on and narrowing of the urethra itself, leading to well known symptoms.
Historically this condition has been relieved by invasive surgery, but the potential side effects of the standard surgery are significant. New treatments for the prostate, however, use selective heating of the diseased prostate tissue to temperatures above 45.degree. C., thereby producing necrosis and resorption. This approach is more likely to avoid damage to the urethra. The principal sources of energy for heating to date include microwaves, RF induction, and RF localized current. See DeFord, Ely, and Fearnot, U.S. Pat. No. 5,304,214, Apr. 19, 1994, "Transurethral Ablation Catheter," Col. 1.
A system for transurethral needle ablation (TUNA.RTM.), produced by Vidamed, Inc., uses RF electromagnetic energy at approximately 465 kHz to heat diseased prostate tissue. The TUNA.RTM. system conducts 465 kHz current from a needle inserted into prostate tissue via a urethral catheter to a ground. This system raises the temperature of tissue adjacent to the needle above 45.degree. C., the temperature is necessary to cause necrosis and resorption of tissue, by resistive heating and by induction. Issa, M. M. and Oesterling, J. E., "Transurethral needle ablation (TUNA.TM.); an overview of radiofrequency thermal therapy for the treatment of benign prostatic hyperplasia," Current Opinion in Urology 6:20-27 (1996). (Another reference states the TUNA.RTM. frequency as 490 kHz. Schulman, C. C. and Zlotta, A. R., "TUNA: A promising new therapy for BPH," Contemporary Urology, October 1995.)
Another system for treatment of BHP, that of Urologix, Inc., applies microwave energy radiated at a frequency of approximately 915 MHz using an impedance matched helical antenna. In this technique, a microwave antenna located within a cannula is inserted into the urethra. When energized, the antenna heats adjacent tissue by induction and molecular excitation, thereby raising the temperature of the target tissue above 45.degree. C. and necrosing the excess prostatic tissue. See, e.g., Rudie, Neilson, and Kauphusman, U.S. Pat. No. 5,326,343, Jul. 5, 1994 (assigned to Urologix, Inc.). For further descriptions of this technique see Mendecki et al., Microwave Applicators . . . . Prostate," Int. J. Radiation Oncology Biol. Phys. vol. 6 No. 11, Nov. 1980, pp 1583-1588; Tadashi Harada et al., "Microwave Surgical Treatment of Diseases of Prostate," Urology, December 1985, vol. XXVI, No. 6, pp 572-576; Rick McClure, "Transurethral Hyperthermia for BPH: Trials goal is to top 80% success," Medical Tribune, vol. 29 No. 9, Mar. 31, 1988 (Thursday), pp 3, 13-14.
In a different application, another system uses RF energy to ablate excess tissue in the mouth, throat, and nose. This system, being developed by Somnus Medical Technologies, Inc., uses RF energy delivered through needles. (See U.S. Pat. No. 5,456,662.) This system operates very similarly to the TUNA.RTM. system described above.
These systems and others like them are showing promise for the tissue ablation tasks for which they are designed. However, various factors limit their usefulness.
Raising the temperature of target tissue sufficient to cause necrosis and resorption requires deposition of significant amounts of RF or microwave energy. In addition, in order to avoid conductive dissipation of the heat generated and collateral damage to adjacent tissue, it is desirable to deposit the microwave energy with a high energy density to produce rapid heating in the target tissue. Given the size limitation for devices, such as antennas or needles, which can be inserted via catheters to deposit RF or microwave energy, it is difficult to generate enough energy density in the target tissue to produce the desired result. Further, needle delivery of conducted RF has the limitation that tissue is destroyed only quite close to the needle. Accordingly, treatments may have to be administered in less than optimal ways or for more than optimal times. It would be desirable to have a means for enhancing the deposition of microwave energy in target tissue while leaving surrounding tissue relatively unaffected.
In addition, the absorption of microwave radiation in tissue is not selective. Radiating microwave energy through the urethra, for example, may cause nearly as much damage to the urethra as to the adjacent prostate tissue. Selectivity in other applications can be a problem as well. Since it is usually desirable to preserve the patient's adjacent non-target tissue as much as possible, this result is less than optimal.
Sievert et al., U. S. Pat. No. 5,197,940, issued Mar. 30, 1993, approached the objective of enhancement using seeding. These inventors first seed the prostate with metallic slivers. The second step is to use an antenna to irradiate the patient with RF radiation at approximately 115 kHz. Sievert et al. recommend frequencies from 50 to 200 kHz and possibly up to 500 kHz. The seeds are tailored to absorb the radiation and heat surrounding tissue. The limitation of this invention is the necessity to emplace the seeds.
It is therefore an object of the current invention to provide an improvement to the method of depositing RF and microwave ablation of tissue, for example for treating prostate hyperplasia, which enhances the deposition of RF or microwave energy in target tissue. It is a further object of the current invention to provide an improvement to said method which selectively enhances energy deposit in target tissue compared to adjacent tissue.